negative control for BrdU

Hello,
A student here is looking at the incorporation of BrdU into lymph node cells.
He tried to compare a peak from anti BrdU-FITC to a peak generated from anti
IgG-FITC (isotype control). After setting the peak from the isotype control
in the first decade, the first peak for the BrdU shifted significantly to the
left. The question is, Is there an appropriate control for BrdU
incorporation, or is one to assume that the first of a bivariate peak is the
BrdU negative population?
Thanks in advance,
Brian R. Tilton
Beth Israel Deaconess Medical Center
Boston, Massachusetts
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